The invention concerns an electromagnetic drive having the characteristics of the preamble superordinate claim 1.
DE 197 120 63A1 or the publication of the corresponding international application PCT/EP 98/01719 1 describes an electromagnetic drive.
The paramount objective in the design of such drives is to achieve the smallest possible losses in the air gap and in the magnetic circuit of the electromagnet and the least possible weight of the moveable mass. In order to achieve said objective, an integration of the armature into an orientable armature lever in accordance with the cited state of the art. Since the laws of physics relate the mass of a rotational system is to the square of transmission, the ratio of the distance of the armature from the fulcrum of the lever to the distance of the point of action on the element to be motivated by the fulcrum was chosen to be less than 1.
The purpose of the invention is to provide a further possibility of reducting the electrical losses of the drive and of the weight of the motivated mass.
The disclosure describes drives in accordance with the cited state of the art but also those drives, whose armatures describe a point to point movements.
The subordinate claims contain advantageous alternative embodiments of the invention.
The minimum one electromagnet described in the disclosure must have at least one active; that is, lift producing, pole.
Preferably, the armature is driven by two electromagnets; however, as will be shown in the following, the drive is also realizable by means of a coil, that practically cooperates alternatively with the different poles. Preferably, the electromagnet or electromagnets are designed as bipolar; however, electromagnets with more than two poles are also conceivable; for example, even pot-shaped magnets. In the case of a bipolar design and orientable bearing of the armature a design is also possible, in which only one of the poles is active; that is, directly effects an attraction of the armaturexe2x80x94thus performs lifting workxe2x80x94while the other pole provides only the return flux over the armature bearing. In the combination of these modalities a solution using one electromagnet and one active pole is conceivable.
The following considerations resulted in the inventive dimensioning of the drive: Principally, the armature mass is determined by the requirements in accordance with maximal drive power. Here, the limiting dimension is the flux density in the magnetic circuit at which saturation occurs. Dimensioning of the armature is determined by the overall yoke breadth and the yoke length. The overall yoke breadth is then again determined by the distance between the two limbs, which is determined in accordance with considerations of magnetic scatter loss. In general, the overall yoke breadth should be kept as small as possible. Optimization of the armature weight is now possible in that the yoke breadth is kept as narrow as possible with the deepest possible yoke depth. In order to minimize the weight, a ratio of yoke depth to the overall yoke breadth comes into play, which is unusual for magnets. Conventionally, magnets are generally dimensioned in such a way that the ratio of breadth to length results approximately in a square. In order to achieve minimal armature weight in the invention, a ratio is selected that is greater than a factor of 1.5, in particular greater than 2 and preferably greater than 3. The result is thus a relatively long, thin armature that must be appropriately mounted.
By dimensioning a long magnet, the magnet can be over-dimensioned in the power balance which has special advantages; for example, for the opening magnet of the exhaust valve or the shutting magnet of the inlet valve, which must overcome the forces of the gas. In the familiar system using an armature lever described above the torsion bar is used simultaneously as the bearing point for the armature lever. In this case, the torsion bar is subjected to an additional flexural load. When dimensioning a long magnet with a correspondingly long armature, according to the invention this is not possible; therefore, pursuant to a further embodiment of the invention, the armature is connected via one or several armature levers to a tube, which is mounted at least on both sides and absorbs the bearing force. The torsion bar can be situated on the inside of the tube and it is completely unburdened by additional flexural forces.
Along with the longitudinal expansion of the valve and the cylinder head the system must be adjustable to the relatively large tolerances of the valve, the valve seat, the cylinder head and the drive housing. To achieve this, it is recommended that the housing is rotatable around the axis of rotation of the armature tube or even around that of the torsion bar or around another axis of rotation away from the armature the housing lies in a bearing pit and is fastened via a cushioning counterbearing. Adjustment is done, for example, by means of two nuts, whereby one nut represents the so-called anvil and is shifted to adjust and the second nut is used for the purpose of securring.
A further enhancement is represented by an arrangement of the magnetic circuit whereby grain-oriented material is inserted, which is economical and reaches saturation in the region of 1.9 Tesla. At the onset of saturation, normal magnet material exhibits a flux density of 1.4 Tesla. Thus, a considerable power increase per unit of area is possible and this results in smaller magnets and reduced armature masses.
A long magnet with high pole area has, however, disadvantages in inductivity and thus in time response; therefore, it is recommended, division of the yoke limb and insertion of two coils. The construction described for the long magnets additionally has the advantage tht the structural width is relatively small, which again permits a relatively low cylinder head. A cost factor is the layout of the coils. Frequently, the yoke is divided when inserting the coils into the magnetic circuit; this means losses at the junctions. In the inventive design the coils are constructed in such a way that they can be installed in the window bwteen the two limbs of the yoke. Correspondingly the maximum width is measured.
A particular problem is presented by the requirements for small time constants with relatively large magnets with corresponding inductivity. A small time constant is required for the purpose of position adjustment and is thus achieved in that the valve is seated with low speed. For this to happen, it is necessary that the magnetic circuit reacts quickly to the respective control signals. This is achieved in that, as described above, by the partitioning of the yoke several coils are used and are switched in parallel. For example, four coils can be provided which can be switched together in parallel. Since these coils, in comparison to one coil, have the same time constants, in less than a quarter of the time the required linkage/permeation is achieved. The job of the magnets is, on the one hand the performance of the lift work for the purpose of the mechanical and the gas losses. On the other hand, a closed or an open valve position should be achieved by the armature in its terminal positions. Over 70 percent of the operating cycle is used for the closed position. In order to keep the required holding energy low the coil current is switched/clocked. However, a separate holding coil can be used. By using said holding coil with the appropriately large number of windings the holding energy; that is, the output, can be drastically reduced. In order to provide for a favorable heat removal the coils are relatively thin and have a relatively large surface thanks to the advantage of the long magnet. In addition, filler pieces between yoke and coil body can be installed for enhanced heat removal. Said filler can be laminated and made of material that has good heat conducting properties but it can also be magnetic material for the purpose of reduction of the ferric [magnetic] losses. There are also possibilities for combining of both methods. The coils are preferably imbedded into the base body and they can in certain cases also be extruded into it.
A large problem is presented by the control of the various longitudinal expansions undergone by the cylinder head and the valve during warm-up. Per the state of the art hydraulic elements are frequently used to even out the play or magnets with large air gap are used. The elements used for hydraulic compensation of play are very expensive and are limited in compensating play, since there is also the risk that the drive is operated outside of its centerline. As in the state of the art described above, an overstroke spring can also be used. With the additional use of temperature compensation in the housing or in the valve, the overstroke is relatively slight; for example, it is limited to a couple tenths of a millimeter and has a less powerful effect on the holding energy at a relative low translation ratio of magnet to valve axis. This overstroke spring has the advantage that at seating; that is, on closing of the valve, generally only the valve mass acts as an impact load or stress. The remaining mass is decoupled by the overstroke spring. Preferably, the overstroke spring is constructed so that the majority of the mass sits upon the small arm of the lever and thus does not directly flow into the effective mass. At the same time the magnet can be brought onto a smaller air gap. The remaining air gap must be dimensioned so that it overcomes the actual valve seal and a temperature expansion without the armature being fully supported. If the armature rests before the valve closes, there would be no valve seal.
There are various possibilities for the transfer of the drive force from the armature to the valve. The least magnetic power and motivated masses and thus also energy requires a direct coupling of the valve to the armature movement.
It is, however, also possible to uncouple the valve via its own, conventional valve compression spring. In this instance the torsion spring and/or a tension or compression spring can provide the necessary counter force. These solutions offer advantages in assembly, but are disadvantageous because of the larger masses motivated, greater magnetic force, and higher energy requirements.
The invention will be described in more detail using the following examples of embodiments.
Wherein: